Noncontact distance measuring sensors known from the prior art which operate on the principle of a variable differential inductor or on the principle of a linear variable differential transformer (LVDT), for example, feature the design-related technical problem that even in the event that the measurement object is situated outside the measuring range, a measuring signal can appear which cannot be distinguished from a valid measuring signal. In a distance measuring sensor or displacement pickup operating on the principle of a variable differential inductor, two coils are connected in series and cooperate jointly with a typically ferromagnetic core as the measurement object in the nature of a Wheatstone half bridge. When the ferromagnetic measuring body, which may be a solenoid plunger arranged in the interior of the coils for example, is situated exactly in the center position between the two measuring coils, the bridge circuit is balanced and the measuring voltage is identically zero. When the core is moved out of this initial position, the impedances of the two measuring coils will change in opposite directions and the measuring voltage will increase within the valid measuring range, ideally proportionally to the displacement of the measurement object. Based on the signal tapped at such a displacement sensor, however, the case in which the core is situated exactly in its initial position, i.e. between the two measuring coils, cannot be distinguished from that case in which the core was removed from the valid measuring range, for example due to a defect of the displacement sensor. For this reason, expensive precautionary measures need to be taken in displacement sensors known from the prior art in order to rule out that the measurement object leaves the valid measuring range.
Similar technical challenges emerge in the case of displacement sensors that operate on the principle of a linear variable differential transformer. Displacement sensors of this type include a primary coil and two secondary coils which are coupled according to the principle of a transformer with a ferromagnetic core which may also be a solenoid plunger. The primary coil, which is situated between the two secondary coils as viewed along a common direction of longitudinal extent of the three coils mentioned, is supplied with an alternating voltage. The core causes a coupling between the primary coil and the two secondary coils. A secondary-side voltage is induced in the two secondary coils that are adjacent to the primary coil, the secondary-side voltage being identically zero in the center position of the core because of the coupling of the two secondary coils in opposite directions. When the core is displaced from this initial position, the measuring voltage will change as a function of the distance covered by the core. This means that also with the displacement pickup operating on the principle of a linear variable differential transformer, it is not possible to distinguish, solely on the basis of the signal provided by such a distance measuring sensor, between that case in which the core is exactly positioned in its initial position between the two secondary coils and that case in which the core has been removed from the coil arrangement. It is therefore required to take respective precautionary measures in such a distance measuring sensor as well, in order to rule out that the measurement object leaves the valid measuring range.
This behavior of the above-mentioned displacement sensors may in some circumstances lead to misinterpretations of the measuring signals provided by these sensors. In addition, the precautionary measures taken to ensure that the measurement object does not leave the valid measuring range involve an additional design engineering expenditure.